Cause A fungus-like microorganism, Aphanomyces euteiches, that overwinters as thick-walled oospores, which can be spread by water, wind, infected plant debris, soil movement, or tillage equipment. Oospores are produced within infected root tissue, which decomposes, releasing the oospores into surrounding soil. The fungus can infect and multiply in other legume roots, such as clover, pea, lentil, Cava bean, and common bean. Even without other hosts, the pathogen can persist in infested soils 10 years or longer, though the population slowly decreases with time.
Symptoms Long, soft, water-soaked areas initially develop on the lower stem and roots that rapidly turn tan and spread throughout the root system. Alfalfa plants with severely infected roots pull out of the ground easily, and the outer portion (cortical tissue) is easily stripped away, leaving only interior tissues. Cortical tissues can be observed with a compound microscope, and diagnostic oospores (25 to 30 microns with an oil globule in the center) are readily detected. Top portions of infected plants are stunted and, in severe cases, wilt, yellow, and die prematurely.
Cultural control
- Long rotations are the best way to manage the disease.
- Green manure plowdowns of oat and Brassica residues has helped somewhat to reduce inoculum levels.
- Select well-drained fields. Avoid fields with excessive soil compaction.
- Avoid fields heavily infested with the pathogen. Test soil to determine inoculum levels in a given field.